In general, known process control devices for regulating the working gap, that is the position of the tool electrode with respect to a workpiece electrode, are based on the principle that a desired signal is compared with a current signal to produce a signal indicative of the deviation, which is used as feedback signal.
Such method is known for instance from U.S. Pat. No. 4,864,091 in which a signal representing the current working gap distance is compared with a signal representing a desired working gap distance and the resulting error between these values is computed to generate a new command value for the drive motor. With this kind of approach, even if the same operation is performed repeatedly and under the same operating conditions, the valuable information incorporated on the tracking error of each repetition gets lost.
Since the type of discharge changes continuously in a stochastic manner, the control continuously corrects the working gap distance. In fact an effective normal discharge may be followed by any other type of discharge (see e.g. Dehmer, J.; diss. 1992 RWTH Aachen, Germany). In summary, conventional EDM process control reflects essentially the instantaneous process conditions. The nominal/current value comparison allows to safely control the process, but the movement of the controlled axis remains essentially bumpy.
Further, considerable time is spent to get a steady process condition after a process pause, in particular after every flushing motion. Besides the inevitable time spent for the flushing motion itself, some time is spent to conveniently approach the erosion front because commutation from a jump speed to an erosion speed must occur at a safe distance to avoid any damage of the electrodes. With elevated flushing motion frequency, the inconvenient is more significant.
An object of the present invention is to provide an improved EDM process control method leading, for example, to a less bumpy movement of the controlled axis and/or a smaller variation of the axis position of the tool electrode. This shall lead to higher productivity, i.e. increase of the material removal rate (MRR), reduction of the electrode wear and improvement of the surface quality and homogeneity.
Another object is to reduce the time to get a steady process after a process pause cycle.